This invention relates to the confinement of field reversed plasma rings and, more particularly, to the steady state maintainance of field reversed plasma rings produced by coaxial plasma guns.
Compact toroidally shaped plasmas (CT's) or field reversed plasma rings having both toroidal and poloidal magnetic field components are presently created with co-axial plasma guns, electrodeless flux-core induction and pinch discharges. These donut shaped plasmas have closed toroidal magnetic flux surfaces produced by currents within the CT. Currents flowing in external, adjacent, conducting walls or magnetic coils are required to maintain radial equilibrium, but external wall and coil structures do not link the plasma as in a tokamak and other toroidal configurations.
Compact toroidally shaped plasmas can be used to inject either constituent particles or stored energy into magnetic plasma confinement devices or as targets for other particle or energy sources. Therefore, CTs are of great interest for fusion reactors. From a fusion reactor standpoint, a CT has a number of advantages including simplification of hardware, including materials design, and size reduction because the closed flux surfaces alleviate the need for current linking structures.
In order to utilize a CT for power production in a plasma confinement apparatus or fusion reactor it is necessary to maintain the CT above the so-called Lawson criteria of 5.times.10.sup.13 particles per square centimeter-sec. and at a sufficiently high temperature (on the order of 6-25 KeV) in order to achieve fusion reactions or interactions. To meet this criteria high density CT's must be produced having a large amount of internal energy or, alternatively, energy and particles added after initial CT formation (or the CT compressed to achieve higher densities). Energy and particle deposition after formation is preferred since the instantaneous power required at formation for higher energy and density CT's places constraints on hardware and is outside of current technology.
Energy in a CT is present in two forms, magnetic energy due to currents flowing in the plasma and thermal energy of the plasma particle species. The magnetic energy is needed to confine the particle species undergoing fusion reactions. The thermal energy (6-25 keV per particle) is necessary to produce sufficient fusion reactions to satisfy the Lawson criteria. The currents flowing within a CT are, however, subject to resistive interaction from the particle species making up the plasma causing ohmic losses or dissipation of the energy stored in such currents. Some of the magnetic energy is continuously converted to thermal energy by ohmic dissipation and unless replenished this leads to loss of confinement and destruction of the CT. The ohmically dissipated energy heats the particle species and is ultimately lost from the CT by particle transport or impurity radiation.
These dissipative mechanisms eventually lead to the loss of internal currents and the closed magnetic flux surfaces of a CT. Therefore, it can be appreciated that it is necessary to add magnetic energy in order to at least maintain a CT in a steady state. It is the main objective of this invention to continuously add magnetic energy to the CT and thus maintain a steady state CT.
There are three basic approaches that have previously been considered for magnetic energy deposition in toroidal plasma devices: injected neutral or charged particle beam currents (such as disclosed in U.S. Pat. No. 4,232,244 to J. H. Fink and A. W. Molvik), currents driven by application of radio frequency waves and currents driven by inductive coupling. Injected particle beam currents and currents driven by radio frequency waves involve complex technologies, and it is not known whether either of these will have high enough efficiency for steady state current drive of a power producing fusion reactor. It is also not presently known whether a radio frequency wave exists that is accessible for driving currents in the interior of a CT plasma.
Inductive coupling techniques have the advantage of simple technology. The energy is coupled to the plasma by transformer action from current carrying means surrounding the CT. Induced currents use the aforementioned ohmic interaction process to resistively heat the plasma. As in a tokamak reactor, inductive coupling will not work for steady state operation since eventually a voltage limit or flux core saturation of the primary circuit is reached. There is an upper limit to the amount of energy that can be transferred to the CT.
These approaches all have technological difficulties and may not extrapolate to the high density, high temperature regime envisioned for CT reactors. In addition, although heating decreases the ohmic dissipation of plasma currents by decreasing the resistivity and prolongs the lifetime of a CT, it will not lead to a steady state, which is desirable from a fusion reactor viewpoint, unless magnetic flux is also added. The magnetic flux is necessary in order to maintain the currents of the CT configuration.
Any current drive or magnetic energy deposition technique will also increase plasma thermal energy via ohmic dissipation of the driven currents. However, particle beams and radio frequency waves may also be used to directly increase the plasma thermal energy without driving the current necessary to maintain the CT configuration. The physics and technology of plasma heating by particle injection and radio frequency waves is more advanced than current drive by either of these methods. Because of the decreasing effectiveness of ohmic heating as the plasma temperature increases, it may be desirable to use particle beams or radio frequency waves for heating the plasma to fusion reactor temperatures, while using the invention described below to maintain the necessary magnetic fields in steady state.